1. Field of the Invention
This invention relates to a device for magnifying and focusing an image. More particularly, the invention relates to a fixed step catoptric zoom system.
2. Description of the Prior Art
Heretofore, zoom optical devices have used multiple refractive lenses or a combination of refractive lenses and reflective elements to achieve selectively magnified images while correcting for chromatic and monochromatic aberrations. Use of refractive lenses in such devices reduces the amount of energy that reaches a detector element and introduces chromatic aberrations. For example, each lens results in energy intensity losses because of surface reflections and because of absorption as the energy travels through the lens material. These losses are typically substantial.
The prior art zoom optics generally work very well in energy frequency bands in which the selected lens materials are transmissive, but these zoom optical devices suffer from the disadvantage of being heavy and bulky. In addition, these devices generally require large lens diameters to increase the amount of energy focused on the detector while compensating for reflection and absorption losses.
In the long infrared wavelength spectrum, lens materials, such as flint glass, are not transparent. Thus, to obtain a refractive system sensitive to infrared energy, lenses made of exotic materials, such as silicon, germanium or sodium chloride, must be used. These materials are somewhat transparent at infrared wavelengths, but reflective losses still occur at each lens surface and absorption losses occur as the energy travels through the lens material. Moreover, infrared optical systems are expensive to manufacture, are delicate and may require special protection when they are used in different environments.
Refractive zoom optical systems are used to provide a change in the field of view and correspondingly a change in the magnification of an image. Since the focal point for two different wave lengths of light is different for a given refractive design, careful attention must be paid to the selection of lens materials and to the figure of the various lens to reduce chromatic aberration to an acceptable level. In addition to chromatic aberration, as the wavelength of the radiation varies, e.g., as the wavelength shortens to ultraviolet and beyond or lengthens to infrared on the other end of the spectrum, energy attenuation as the incoming radiation passes through lens elements becomes important. It is, in general, not possible with currently known lens materials to use a single material with ultraviolet, visible and infrared parts of the spectrum.
A reflective device provides two major advantages over refractive designs. First, reflective elements are free of chromatic aberration. This permits all wavelengths from ultraviolet to infrared to be focused at the same point. Secondly, although the reflectivity of mirror coatings may change with wavelength, a mirror is not subject to absorbtivity since the electromagnetic energy does not pass through the material.
The subject invention provides a zoom optical device, capable of selectively focusing and magnifying in fixed steps the image of an object emitting energy in wavelengths from ultraviolet through infrared, without the many disadvantages in cost and construction of the prior art devices.